Re-Inspect that ground strap off the motor, do a continuity test between your motor and one of the bolts on the strut tower. If its oxidized enough wire brushing it will just prime it for the next layer of "rust" to get on there and render it unconductive sooner then you'd think. Also when I say "conductivity test" that is the basic test for "there is a connection" since the starter draws a high amount of current it also needs a good solid return. Just because it beeps doesn't mean its legit. Read the Ohmic Resistance Value from one end to the other of the strap and post it here. It should be very very minimal, considering they are using copper wire to make the strap its crucial that it has a very low resistance. You may need to purchase a new one or buy a insulated piece of thick copper wire crimp two rings on each end and rebolt it down as your new ground.